1. Field of the Invention
The invention involves a process and a curtain-coater for coating a base by means of a free-falling liquid curtain comprising a coating solution. This process has long been known as the curtain-coating process. In this process, a base in the form of a web is moved continuously by a transport device through a coating zone and is thereby coated with one or more layers either wholly or partially by the free-falling liquid curtain.
2. Description of the Related Art
In the photographic products industry, this process is used, for example, to apply photosensitive and photoinsensitive coatings. These coatings comprise mostly multiple layers formed from aqueous coating solutions, which are coated as layer composites in the liquid state onto the base. The base is mostly a synthetic resin film one to two meters wide or a paper web. The coated base then passes though a drying device in which the coating solution is dried. The dry film web is wound up. At this point, the edges of the web must be dry or else the individual layers of the roll will adhere. During coating, a troublesome beaded edge forms most of the time and is vacuumed away immediately after coating.
In the curtain-coating process, a coating head, called coater for short, prepares the layer composite. The coater has an inclined slide surface containing coating slots. The coating solution issues from the coating slots and flows along the slide surface to a so-called coating lip. The layer composite leaves the coating lip and falls freely as a liquid curtain onto the base.
The process speed is limited primarily by breaks in the liquid curtain. Surface tension tries to narrow the curtain. This is prevented by so-called curtain holders, which guide the curtain at its edges and keeps it spread during the free fall. These curtain holders are also called edge guides, side guides, or side holders. The speed of the liquid flow in the margin layer next to the curtain holder is lower than the speed in the middle of the curtain. The margin layer near the curtain holders becomes displaced. The flow lines near the curtain holders do not run vertically straight but are curved. This results in the liquid curtain having areas that are thinner than in the middle. These areas are also called areas of "contraction". They are narrowed areas in the liquid curtain, they run inclined to the curtain holder, and at the lower end of the curtain holder are spaced apart by four to eight mm. They are the weak spots in the curtain. When the coating process fails, the curtain breaks predominantly at these areas of contraction. One cause of this breaking action is the flow at the lower end of the curtain holders. Turbulence originating in this area spreads to the above-cited weak spots and leads to breakage of the liquid curtain. Attempts have been made to reduce this undesired displacement action. For example, the curtain holder is wetted with an auxiliary liquid. This auxiliary liquid is thinner than the coating solution. The auxiliary liquid lubricates the area of the curtain bordering the curtain holder and thus reduces the speed differential. The shape of the curtain holder, its geometry, and the selection of construction material also influence flow behavior in the margin layer. Various curtain holders are known in the current state of the art, for example, a wire form in U.S. Pat. No. 5,328,726, a bar form in U.S. Pat. No. 4,830,887, a flat form in U.S. Pat. No. 4,135,477 and GB 1 518 552, and a porous circular form in EP 0 115 621.
The curtain in the curtain-coating process can be wider or narrower than the base. EP 0 425 562 describes a curtain-coating process in which the liquid curtain is wider than the base. A separating device separates a curtain edge on both sides and removes the edges from the coating zone. The separating device comprises essentially a flat, cantilevered blade. This blade projects from a vacuum housing and interrupts the free fall of the curtain in the immediate vicinity of and parallel to the base. The interruption occurs just before the curtain lands. The blade is thin and sharp. It is rinsed on its upper side by a cleaning liquid. The stream of cleaning liquid rinses the liquid of the curtain edges out of the coating area. If the curtain edges comprise a gelatin solution, only part of the valuable coating solution is lost, but this process has many disadvantages. One is a crust that accumulates on the edge of the blade during long operation. This is caused by gelatin residues. Thus, the blade becomes dull. A dull blade cannot satisfactorily prevent a beaded coating on the edge. An improved embodiment of this invention, in which a cleaning liquid rinses the underside of the knife, also has disadvantages. Turbulence in the liquid flow at the lower ends of the curtain holders is not adequately isolated from weak spots in the curtain. Furthermore, flow on the underside of the blade is always unstable. Fundamentally, the cantilevered, sharp-edged blades pose an ever-present risk to the operators. Cleaning the blades can result in injuries. The thin blades can be easily bent and damaged. This can lead to stoppages in the coating operation.
EP 0 606 038 discloses a curtain-coating process in which the liquid curtain is narrower than the base. The curtain edge falls on a projecting flat blade and is vacuumed away from there. Only the middle section of the free-falling curtain lands on the base. The flat blade must also be very thin to achieve the desired improvement in the edge of the coating. Only a sharp thin blade positioned just above the base minimizes the beaded edge of the coating. Here, too, the edge of the blade also becomes encrusted during operation. Therefore, the coating becomes uneven at the edge which results in turbulence.
The fundamental goal in film manufacture is to use material as efficiently as possible. The base should be coated as uniformly as possible up to the edges. Furthermore, the manufacturing process should not be susceptible to defects during the production time for one batch, which can last, for example, up to one day. The curtain-coater should present the least possible risk for operators during equipment cleaning.